1. Field of the Invention
The present invention relates generally to an automatic controller, and more particularly to an automatic controller which automatically controls the subject of control by a motor, and which enables control by external forces such as hand operation.
2. Description of Related Art
There is a conventional automatic controller which controls the subject of control so that the position and speed, etc. thereof can follow arbitrary changes in desired value. The automatic controller is applied to drive a zoom lens of a TV camera for example. To drive the zoom lens by a motor, the automatic controller makes the rotational speed of the motor follow changes in desired value which is input by a final controlling element, and it moves the zoom lens at desired speed.
If a disturbance (external force) effects the controlled variable such as the position and speed of the subject of control such as the zoom lens, the above-mentioned automatic controller detects the controlled variable and performs the negative feedback in order to reduce the effects of the disturbance. If the positive feedback were performed, the subject of control works, on detection of the disturbance, to increase an error even without the operation of the final controlling element. In this case, the subject of control is uncontrolled.
As stated above, the automatic controller is composed of the negative feedback to avoid the effects of the disturbance such as load changes on the controlled variable. Depending on the situation, however, some changes may be desired with respect to the controlled state. In this case, the operation using the external forces such as hand operation is sometimes more favorable than the operation of the desired value for control. For example, in the case of the automatic mode wherein the motor drives the zoom lens, it is more convenient if a user manually drives the zoom lens so that the zoom lens can be finely adjusted.
According to a conventional automatic controller, the hand operation is impossible unless the control of the automatic controller is cancelled. When an operator starts operating the subject of control manually in the automatic mode, the mode must be switched to the manual mode, and thus, the switching operation requires a lot of trouble.